1. Field of the Invention
The present invention relates to a method of producing a ground component of silicon nitride ceramic, and more particularly to a method of adding strength to a processed surface thereof.
2. Description of the Prior Art
Silicon nitride ceramics have attracted attention as a material for a mechanical part used under extremely severe conditions because of their lightweight and superior wear resistance, strength and high-temperature strength thereof. Since the mechanical parts used under such circumstances are required to be processed with a high precision and a high level of quality, grinding with diamond wheels are typically used to finish the part to a desired shape.
However, it has been reported that cracks parallel to the grinding direction are formed in the processed surface during the grinding process, which makes the inherent strength of the material remarkably degraded (see, for example, Imai et al., "Toyota Tool and Machine Technical Report", Vol. 26, No. 3.4, Pages 25-26).
On the other hand, residual compressive stresses are generated in the ground surface of the material along with the cracks formed therein, restraining degradation of the strength. The residual compressive stresses are, however, insufficient to restore the strength because of the grinding crack having greater influence on the degradation of the strength. It has also been reported that the residual compressive stresses in the processed surface is reduced by heat treatment at 1200.degree. C. for 2 hours in a vacuum (see, Suzuki et al., "Material", Vol. 38, No. 429, Pages 584-585).
Therefore, it is considered very important to restrain any damages on the material surface as much as possible in grinding ceramic components. To satisfy this requirement, the grinding was performed with diamond abrasive grains of a small grain size. However, the grinding with the diamond abrasive grains of the small grain size was not always an effective and preferable choice from the viewpoint of productivity and cost.
Studies have thus been made to improve the strength of the ground surface in grinding ceramic components, and various methods have been proposed and practiced. For example, Japanese Patent Laid-Open No. 4-243988 discloses a method for strengthening a surface of a ceramic base comprising coating the surface of the ceramic base with a different kind of oxide ceramic having a large thermal expansion coefficient, firing the coated ceramic and cooling the same to provide a residual compressive stress in the surface of the ceramic base by using the difference in thermal expansion coefficient. It is, however, pointed out that this method complicates the production process and is thus insufficient from the viewpoint of the cost and the productivity.
Japanese Patent Laid-Open No. 2-167864 discloses a method of improving strength by the effects of crack healing and increased residual compressive stress comprising finishing a processed surface of a ceramic to have a roughness of 1-3 .mu.m measured by the maximum surface roughness (Rmax), heating it at 1000.degree.-1300.degree. C. to form an oxide film, getting the growth of the grinding crack dull, and cooling the surface from a temperature of 400.degree.-500.degree. C. at a rate of 50.degree. C. per minute or higher to generate the residual compressive stress in the treated surface. However, it is pointed out that this method requires two steps of heat treatment, has some limitations on cooling equipment, and is less reliable because not all grinding cracks are subjected to the healing effect by the first heat treatment step.
Alternatively, some attempts have been made to heal the crack in the processed surface formed during grinding with an oxide film by means of heat treatments at various temperatures and in various atmospheres, thereby improving and ensuring the quality level of the material. For example, Japanese Patent Laid-Open No. 60-81076 discloses that Si in the ceramic is reacted with oxygen in the air by keeping a silicon nitride ceramic at 950.degree.-1400.degree. C. for 30 minutes or more, thereby producing an oxide which eliminates defects such as scratches and cracks formed in the material surface, improving the flexural strength.
Japanese Patent Laid-Open No. 1-52679 discloses that, as a method for improving strength of sialon, the material heat-treated in a temperature range of 800.degree.-1100.degree. C., especially 900.degree.-950.degree. C., in the air would have a strength higher than defect-free materials before heat treatment, and that the reason therefor may be the oxidation of the material surface and diffusion of, for example, Al in the grain boundary phase to cover the surface with a generated layer, thereby reducing the defects such as fine grinding scratches or the like.
Japanese Patent Publication No. 3-80755 discloses that a silicon nitride sintered body including Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, and AlN as sintering aids is ground into a predetermined shape and then heat treated in the air at a temperature lower than the sintering temperature but higher than the softening temperature of the glassy phase, namely at 800.degree.-1100.degree. C., for 1-24 hours so that micro cracks generated during the grinding process are repaired and an oxide film of SiO.sub.2 is formed on the surface. Due to the synergistic effect of these, the mechanical strength is improved.
Japanese Patent Laid-Open No. 5-149112 discloses that, as a method of manufacturing a valve of silicon nitride ceramic comprising of 75-89% by weight of Si.sub.3 N.sub.4, 1-5% by weight of Al.sub.2 O.sub.3 and 10-20% by weight of Y.sub.2 O.sub.3, the grinding cracks can be eliminated to provide a valve of high reliability when the ground surface is processed to have the maximum surface roughness (Rmax) of preferably 1-3 .mu.m or less and then heat treated at 1000.degree.-1300.degree. C. for one hour in the air, or in an atmosphere of nitride gas or argon gas.
Japanese Patent Publication No. 4-50276 discloses to heat a sialon sintered body at 875.degree.-950.degree. C. for 30 minutes or more in an atmosphere including oxygen and to treat the surface with a glassy layer such as SiO.sub.2, Al.sub.2 O.sub.3, and Y.sub.2 O.sub.3 for filling defects in the surface, thereby improving the corrosion resistance and strength.
Further, Japanese Patent Laid-Open No. 4-36759 discloses a silicon nitride sintered body with an oxide film, whose resistance to defect and wear resistance are improved by eliminating defects remaining in the surface. More specifically, a sintered body including at least one of .alpha.-Si.sub.3 N.sub.4, .beta.-Si.sub.3 N.sub.4, .alpha.'-sialon, and .beta.'-sialon is heated at 800.degree.-1100.degree. C. in an oxidative atmosphere to form an oxide film (oxide, oxynitride, oxycarbide, oxynitride carbide) of 10,000 angstrom or thinner.
All the methods described in these documents are only to improve the reliability through the heat treatment under the conditions of predetermined temperature, time and atmosphere with the resultant healing effect caused by the oxide film and a glassy layer of the assistants. As apparent from a degree of improving the strength described in examples, the effect is not sufficiently ensured from the industrial viewpoint.